Solar power is highly useful for applications where long-term, remote operation is desired, such as lights, road signs, security cameras, and so on. Since some of those devices need to operate at night as well, a battery to store the solar energy accumulated during the day is useful.
One problem associated with batteries, particularly lithium batteries, is that they typically do not charge very well in low temperatures, rendering them less useful for outdoor applications in cold climates. While a battery can still release power in low temperatures, it cannot be charged in such conditions. This means that the solar energy released on a cold, sunny day would be wasted, and the device would not be able to operate at night—even though if the battery were fully charged, it could operate just fine.
For example, the temperature range in which a NiMH battery can be charged is 0° C. to 45° C.; the temperature range in which it can be discharged is −20° C. to 65° C. The ranges are similar for Li-ion batteries; the temperature range for charging is 0° C. to 45° C. and for discharging is −20° C. to 60° C. While temperatures in excess of 45° C. are uncommon, temperatures below 0° C. are extremely common in the winter in some locations, and it is also quite common for temperatures to not rise to 0° C. at all on a winter day; thus, an outdoor solar-powered battery would not be able to be charged at all on those days, even though it could be discharged.
A need exists for a solar-charged battery that can operate in a wide range of temperatures and that can be optimally charged even when the ambient temperature is low.